Generally, concrete shrinks as it sets. The shrinkage is small and like concrete strength it is about 80% complete in 30 days.
This is generally understood in the construction industry and several methods have been employed to deal with it. The most common method is to saw cut the concrete slab as soon as it can support a worker. The long accepted method, is to saw to 1/4 of the slab thickness at intervals of about ten to twelve feet. This does not stop the cracking but keeps the cracks in straight lines. If floor coverings are required, the saw joints can be filled at a later date. This is generally successful with pavements but less successful inside buildings.
The most common problem in buildings is the columns supporting upper floors and/or roof. Cracks frequently form off of the corners of rectangular columns. Designers want the joint pattern on column lines, parallel to the walls and the sides of the columns parallel to the walls.
Experience has shown that unless concrete slabs are sawn to any included inside corners, such as column pedestals, stairways, pits etc., cracks will generally start at the corners and proceed randomly across the slab. Even sawing very near these locations, is still frequently unsuccessful at controlling these cracks.
Industry practice is to form diamonds around rectangular columns. This practice rotates the column corners 90 degrees to put them in alignment with column center lines while isolating the floor from the foundation. Joints are then sawn on column lines. This practice requires separate forming and concrete pouring operations for the diamonds. The final result usually controls cracking, but leaves an extra joint around each column and some minor elevation differences between diamonds and floor. These areas require an additional leveling operation before installing floor coverings and then slight movement in these joints results in cracks or bumps in the floor coverings.
Another practice, frequently utilized around large floor openings, is to add a couple of short pieces of reinforcing steel diagonally across the corners. This practice is less effective in eliminating corner cracks. However, it does tend to minimize excessive movement in the crack.
There has continued to be a problem for satisfactory crack control in concrete slabs or floors and particularly around columns.